The invention relates to a display device comprising an electro-optical medium between two parallel transparent substrates having drive means to influence the transmission state of the electro-optical medium, one of the substrates having a pattern of at least two luminescent materials, said display device comprising a radiation source suitable for emitting sufficiently short-wave radiation to excite the luminescent materials.
The said display devices are used, for example in colour television or in colour monitors for the purpose of data display in computer systems, and in, for example display devices in dashboards etc.
A display device of the type mentioned in the opening paragraph is described in British Patent Specification No. 2,154,355. In the device shown in this specification a liquid crystal display device consisting of a liquid crystalline material between two glass plates is driven in the transmission mode. The device is exposed with UV radiation on one side, while the glass plate on the side of the UV source is coated with a pattern of phosphors. The areas associated with this pattern can be separately switched by means of switching electrodes.
The ultraviolet radiation emitted by the UV source produces conversion in the luminescent layers into a colour in the visible part of the spectrum (for example, into the primary colours red, green and blue) so that visible light passes to the other side of the device in dependence on the state of the electro-optical medium to form a (colour) picture.
However, in this conversion a large part of the quantity of light generated in the phosphors is lost. In fact, the conversion is effected within a very thin layer (approximately 2 to 3 microns) on the side of the incident ultraviolet radiation. Since the generated visible light is emitted in all directions and is also dispersed by the phosphors, a large part thereof (approximately 60 to 70%) leaves the phosphor layer on the side of the UV source. This leads of course to a lower brightness, but moreover a part of the light generated in the phosphors may be dispersed back via reflection from various surfaces (for example a glass-air surface when the luminescent material between the UV source and the electro-optical medium is provided on a glass substrate), and then at an unacceptably large spatial angle or in undesired areas. All this leads to a loss of resolution and a decresed contrast.
It is an object of the invention to provide a display device in which the said drawbacks are at least substantially obviated. It is based on the recognition that notably the said loss of light due to emission and dispersion in the direction of the radiation source can be substantially prevented.